Hydromechanical, split torque, or parallel path powertrains provide many advantages over the typical mechanical transmissions used in earth-working machines, such as tractors, bulldozers, and wheel loaders. Specifically, hydromechanical transmissions provide continuous speed control, control of acceleration and deceleration, and management of engine speed with fewer losses.
Current industry practice with hydromechanical transmissions is to control speed by controlling pump displacement. This practice requires a servo-feedback control on a pump actuator to force the pump actuator to a specific position. Industry has had a measure of success with speed control systems in the agricultural industry but no known successes in the earth-moving industry. The lack of force (torque) control put the earth-working machines with hydromechanical powertrains at a disadvantage when compared to conventional powertrains, particularly when the machine was pushing against massive objects, or digging. Similarly, it was found that because earth-working machines are often employed on rapidly changing surface conditions and compromised stability, it was inherently harder to make smooth speed transitions with the traditional speed control system.
One type of system for controlling speed is discussed in U.S. Pat. No. 6,684,636 to Smith. Smith teaches a method for controlling speed using an electrical signal applied to a solenoid to change the pump's displacement. On generally level surfaces, this method has been successful, however, as discussed above, on uneven surfaces, an operator, or machine experiences undesirable accelerations as the controls hunt for the desired speed.
The inability to determine the correct pump displacement under rapidly changing surface conditions has resulted in the unsuccessful use of the speed control system in the earth-moving industry. Any error in the displacement showed up as lugs and lurches and generally unfamiliar machine behavior when compared to the precedent established by past generations of successful earthmoving machines.
The present invention is directed to overcoming one or more of the problems as set forth above.